1. Field of the Invention
The present invention relates to a distortion-compensating apparatus; particularly, the invention relates to a distortion-compensating apparatus useable for a transmission-use RF-power amplifying circuit used in a portable telephone.
2. Description of the Related Art
Recently, communication has been performed for an increasing amount of data at an increasing speed, and corresponding linearities are strictly required of transmission-power amplifying circuits in digital radio communication apparatuses. On the other hand, however, problems arise in that the strict linearity requirements impair the improvement in power efficiency of power amplifying circuits.
Time for continuous communication using a digital portable telephone already enjoying wide use in the market is increasing. For a new digital radio communication apparatus to hit the market, in view of competitive power of the product, use time thereof cannot be ignored. Recently, there is a growing tendency that a new distortion-compensation technology is employed for the apparatus to improve the efficiency in communication.
The employment of the technology, however, entails voluminousness in terms of the size of the circuit, so that the technology can not be put to use in portable telephones having an advantage of being small sized and light in weight. In addition, since the portable telephones are characterized in that they are used in variable environments, the distortion compensation needs to be adaptable to the environmental variation. The adaptability coupled with miniaturization becomes a very important problem. As a distortion compensation apparatus of this kind, there is known a predistortion technology that is provided with compensation means having a reverse characteristic to the distortion of the power ampifier circuit.
There are a number of reports regarding the aforementioned predistortion technology, including reports on such as the adaptation of predistortion as well as feedforward and the like. Hereinbelow, a description will be made regarding a couple of conventional examples in the field of the adaptive distortion compensation apparatuses used for the aforementioned predistortion.
As a first conventional configuration, description refers to a report on xe2x80x9cPower ampifyingAdaptive Linearization Using Predistortion with Polynomialxe2x80x9d, 1992-European Microwave Conference, Vol. 22, pp.1125-1130. FIG. 11 shows a block diagram of the example introduced in the document.
In FIG. 11, Vout=A(Vin) represents a nonlinear-type output characteristic of a power amplifying circuit 12 for which distortion compensation must be performed. A linearization comparator circuit 50 performs arithmetic operations of in-phase and orthogonal signals I and Q by using a function H (I, Q) that linearizes A (Vin). As a result of the arithmetic operations, the signals I and Q are fed to a digital-to-analog converter circuit (DAC) 51, are converted to analog signals, and are concurrently converted to radio-frequency(RF)-band signals. The converted signals are then input to the power amplifying circuit 12. An output Vout therefrom is detected and is converted by a demodulator circuit 52 to thereby produce base-band converted signals If and Qf. As adaptive compensation relative to variations in temperature, the linearization comparator circuit 50 performs comparison between the signals I and Q and the detection signals If and Qf; and the constants included in the linearization constant H are adjusted so that the difference between the signals becomes zero. The above-described operations are repeated until the difference precisely becomes zero, thereby finally determining the constants included in the constant H (I and Q) to be optimal values.
Another conventional configuration is described in the document xe2x80x9cAdaptive Linearization Using Predistortionxe2x80x9d, IEEE Transaction on Vehicalar Technologies, Vol. 43, No. 2, May 1994, pp. 323-332. FIG. 12 is a block diagram shown in the above-referenced document. In FIG. 12, the same reference numerals and symbols as those in FIG. 11 are used for corresponding portions for the convenience of description. In response to input signals I and Q, a conversion table 55 in a memory or the like is accessed to perform data conversion. As a result, data Ixe2x80x2 and Qxe2x80x2 that can be used to linearize a power amplifying circuit 12 are obtained, and are input to a power amplifying circuit 12. An output Vout therefrom is detected and is converted by a demodulator circuit 52 to produce base band converted signals If and Qf. As adaptive compensation relative to variations in temperature, the circuit comprises an address generator section 54. The address generator section 54 performs comparison between the signals I and Q and the detection signal If and Qf and to adjust an address for accessing the conversion table 55 so that a difference en between the signals becomes zero. The address generator section 54 is repeatedly adjusted until the difference en precisely becomes zero to finally optimize the value of the address used to access the conversion table 55.
In the above-described conventional configurations, optimization is performed for either the constants included in the linearization function or the address used to access the linearization table. In either of the examples, a feedback loop is used to repeat the required operations and to thereby reduce the intersignal difference. However, the feedback loop has serious problems in that it includes characteristics of the power amplifying circuit 12, and there is no way of ensuring that optimal values are stably obtained at all times.
The present invention is implemented to solve the above-described problems. An object of the invention is to solve the problems by detecting input/output in a power amplifying circuit, thereby detecting deviation from a linear gain, and performing distortion adaptive compensation relative to variations in temperature. The present invention can easily provide a distortion-compensating apparatus that solves the conventionally caused problems in which optimal values cannot be obtained.
According the present invention, a distortion-compensating apparatus for detecting envelope signals of an RF (radio frequency) input signal generating envelope variations to thereby perform predistortion for controlling amplitude characteristics of power amplifying means for which distortion compensation is to be performed is characterized in that envelope signals at an input side and an output side of the aforementioned power amplifying means are individually subjected to arithmetic operations, the amount of deviation from a linear gain is thereby obtained, and distortion-compensating data in a storing means which prestores compensation values for amplitude compensation is updated to data obtained from the arithmetic operations so as to be adaptive to temperature variations in the aforementioned power amplifying means.